A typical aircraft synchronous generator includes a permanent magnet generator (PMG), an exciter generator, and a main generator, all on a common shaft which is driven by the aircraft engine. The PMG is used in connection with a voltage rectifier and regulator for developing field power to the exciter. Next, the exciter generates three-phase alternating current (ac) power, which is then rectified and applied as the direct current (dc) field power for the main generator. Finally, the main generator develops the three-phase AC output power for the aircraft.
A major concern in the aircraft industry is to find an efficient method for starting turbine engines. Various systems have used the main generator configured as a motor to drive and start the engine. In order to operate the main generator as a synchronous or brushless DC motor, it is necessary to apply dc power to the main generator field winding. However, since in a typical dynamoelectric machine of this type the main field winding is part of the rotating member, some means must be devised to transfer excitation power to the rotor, from the easily accessible stator. The approach of using a slip-ring configuration is not desirable, due to brush wear and carbon dust considerations. Furthermore, a conventional exciter generator cannot be used in this application, since the dc field power must be initially supplied when the engine is not running, and a conventional exciter cannot develop power if its rotor is not rotating.
It is possible, however, to utilize a rotary transformer, having its secondary winding placed on the common shaft, to couple externally-generated power to the main generator field winding, even while the machine is at a standstill. A rotating transformer would utilize, high-frequency ac power, supplied by an external ac-to-ac converter, to energize its stationary primary winding, which would induce an ac current in its secondary winding. This ac-coupled power could then be rectified to provide the dc field power to the immobilized main generator rotor. Such a rotating transformer configuration would require the use of an additional device in the power system coupled to the common shaft. The use of an additional rotating transformer device for starting the engines would not only increase the weight of the aircraft, but also occupy valuable space.
Another approach was taken in U.S. Pat. No. 4,743,777, wherein a multiple-phase field winding was included with the dc field winding in the stator slots of an exciter. Such a construction, however, exhibits substantial pole-to-pole flux leakage between adjacent slots which create the magnetic poles. When using the dc field winding in its exciter generator mode, this flux loss tends to significantly degrade the efficiency of the exciter.
A need, therefore, exists to provide an improved engine starting technique which addresses both the size and weight considerations, as well as the efficiency requirements.